The proposition, which sought to deny the federal government the authority to penalize people for not getting insurance, is expected to have little practical effect on implementation of the healthcare law.

But the Missouri measure represented the first electoral test for the landmark legislation that Obama signed in March. And it underscored continued hostility to the law from Republican voters.

Tuesday's vote was held in conjunction with the state's Democratic and Republican primaries, but GOP voters were expected to dominate the balloting because their primary included several contests that stirred greater interest.

In November, voters in Oklahoma and Arizona will also consider amendments to their state constitutions that would attempt to head off the insurance mandate.

"There is just a backlash against everything Washington right now," said C.C. Swarens, executive vice president of the Missouri State Medical Assn., which backed the Missouri ballot initiative, known as Proposition C. The group was among several state doctors' groups that diverged from the American Medical Assn., which supported the federal healthcare law.

Opposition to the law has remained particularly intense among Republicans, with nearly 8 in 10 in a recent national USA Today/Gallup survey saying it was a "bad thing."

Public opinion in general remains divided on the law, with several recent surveys suggesting that support may be inching up even as substantial numbers of Americans still favor repealing all or part of it.

But many healthcare and legal experts expect that the real challenge to the law will be settled in the courts. GOP officials in 21 states, including Missouri Lt. Gov. Peter Kinder, are pursuing a series of legal challenges, arguing that the insurance requirement represents an unconstitutional expansion of federal government power.

One of those lawsuits — by Virginia Atty. Gen. Kenneth Cuccinelli — cleared an early hurdle this week when a federal judge denied a move by the Obama administration to have it dismissed.

Supporters of the healthcare law, including many policy experts, think a mandate with fines for people who do not comply is vital to preventing healthy people from purchasing insurance only after they get sick, thereby pushing up costs for everyone.

But as midterm congressional campaigns heat up, the administration and its allies were wary of raising the profile of a political battle they anticipated they would lose.

And leading backers of the healthcare law, including senior Democrats, consumer advocates and national medical groups, steered clear of the Missouri ballot fight.

The Missouri Hospital Assn., which opposed Proposition C, spent more than $400,000 on mailers that warned that eliminating the mandate could push up costs for hospitals and others who must pay to care for those without insurance.

"We understand that the issue is very controversial," said Dave Dillon, a spokesman for the association. "We thought what was important was to educate voters."